Brewer's yeast is used as a biocatalyst to promote fermentation of carbohydrates to ethanol in the production of beer and other fermented beverages. In brewing, fermentation is performed by mixing brewer's yeast with a source of fermentable carbohydrate (usually wort) and incubating the mixture under conditions suitable for fermentation.
Commonly, wort is inoculated or “pitched” with yeast harvested from a previous fermentation. Generally, pitching yeast has experienced anaerobic conditions during fermentation. Before a subsequent fermentation can occur, the anaerobic pitching yeast must first synthesize essential lipid components, including sterols and unsaturated fatty acids. Synthesis of these lipids requires oxygen and a sufficient reserve of glycogen.
A conventional approach to insuring sufficient oxygen for yeast to synthesize lipids has been to aerate the wort. However, by this method, the level of oxygen in the wort must be carefully controlled to ensure a consistent product. Sub-optimal oxygen levels cause slow fermentation. High levels of oxygen in the wort can cause rapid fermentation, which leads to alterations in the beer flavor and overfoaming.
U.S. Pat. No. 899,756 discloses a method of enhanced fermentation that includes aerating or oxygenating wort after pitching with the yeast culture. Aerating the wort after pitching is routinely done in the brewing industry to promote fermentation. However, it is thought that aerating the yeast in wort may contribute to the production of staling precursors.
Attempts to develop ways of aerating or oxygenating yeast slurries prior to pitching have been made. An important consideration in developing such methods is how to deliver oxygen to the yeast at a rate that approaches the rate of oxygen uptake by yeast. A dense brink yeast slurry for pitching has an oxygen uptake rate (r) of about 5 mg/min/liter. If the slurry were saturated with air, the dissolved oxygen level would be about 10 mg/liter. The yeast in this slurry can take up this oxygen in 2 minutes.
Several systems described in literature were devised that supply oxygen at a rate sufficient to meet the high oxygen demand of yeast.
UK Patent Application GB 2 197 341 discloses a method of aerating yeast prior to fermentation in which the pitching yeast is first diluted with water and then exposed to oxygen for a period of time until the yeast reaches its maximum rate of oxygen consumption. The yeast is then used to pitch oxygen-free wort. In this system, oxygen is supplied through a fritted gas distributor. Oxygen thus supplied enters the slurry in the form of small gas bubbles that cause foaming, especially at higher supply rates, which increase bubble size. To reduce foaming, the yeast slurry was diluted 10-fold, thereby reducing the oxygen requirement by 10 times.
Belgian patent Be 1010885A3 discloses a method of oxygenating yeast by means of a micro-porous ceramic membrane contactor through which a dense yeast slurry is circulated. The aluminum oxide membrane has an average pore size of 0.5 micron. In order for the oxygen to pass through the membrane, a higher pressure must be maintained on the gas side of the membrane, relative to the pressure on the liquid side of the membrane. Under these conditions, oxygen enters the yeast slurry as micro bubbles. In this system, formation of foam is reduced but not eliminated. Because of the smaller bubble size and the large number of pores per unit surface area, this method affords improved mass transfer of oxygen to the yeast slurry over that of the method disclosed in GB 2 197 341. However, mass transfer is not optimal because oxygen supplied in the form of bubbles is slow to dissolve because the bubbles move along with the liquid, causing a relatively large liquid boundary layer and a slow dissolution rate. Delivery of sufficient oxygen to the yeast by this method would depend on providing a very large contactor made of relatively expensive ceramic material.
U.S. Pat. No. 5,565,149 discloses a method for delivering oxygen with a membrane system that supplies oxygen without forming bubbles. The membrane is 99.9% non-porous. Oxygen is transferred across the membrane by diffusion. The efficiency of oxygen transfer and the oxygen transfer rate are reduced by the additional diffusion step.
Oxygenating the yeast prior to pitching is known to be superior to oxygenating the wort in terms of producing a consistently good product. Despite this, yeast oxygenation prior to pitching is not generally practiced in breweries because of difficulties associated with supplying oxygen to pitching yeast at a rate sufficient to keep up with the very high oxygen uptake rate of the yeast. Those systems described in literature that report high oxygen supply rates are uneconomical or cause excessive foaming.
There remains a need in the art for an economical method of aerating or oxygenating that does not cause foaming.